The invention relates to polymer dispersions, also known as latices, particularly polymer dispersions which are electrically neutral or mildly anionic that can be used, e.g., to produce anionic or cationic polymer dispersions.
Polymer dispersions or latices consist of small particles of polymers, typically ranging in size from 60 nm to 250 nm, dispersed in water. They are typically produced using emulsion polymerization, and can be used in a wide range of industrial applications, including paints, paper coatings, seal coatings, waterproofing membranes, adhesives, carpet backing, printing inks, non-woven fabric, leather finishing, dipping goods, asphalt and concrete modifications, medical applications, and the modification of plastic materials. Styrene-butadiene copolymers, polyacrylates, and vinyl-acetate polymers account for 95% of the total production of polymer dispersions worldwide. When dried at temperatures above the polymer dispersion's minimum film-forming temperature, polymer dispersions form a polymer film that can be clear or opaque, hard or tacky, and plastic or elastic, depending on the particular properties of the polymer dispersion. Though a polymer film may not be visible after drying, it often provides critical properties to the end product.
Most commercial polymer dispersions are negatively charged due to the presence of anionic surfactants and copolymerizable vinyl acids added to the recipe during emulsion polymerization. This process of producing anionically charged polymer dispersions is often referred to as carboxylation. Carboxylation is extensively used to produce anionically-charged polymer dispersions used in wide range of applications, including paper coating, paint, carpet backing, and adhesives.
In some specific applications, such as paper making, paper sizing and emulsion-based asphalt paving, cationically-charged, rather than anionically-charged, polymer dispersions are desired. One known method of producing cationically-charged polymer dispersions uses a cationic surfactant, a cationic polyelectrolyte, and a small amount of monomers with cationic functional groups which can be co-polymerized together with other major monomers, such as styrene, diene and acrylate derivatives, to produce the polymer dispersion. However, production facilities using this method of producing cationic polymers must be isolated from production facilities that produce anionic polymer dispersions, because cross-contamination of the two differently charged polymer dispersions can cause immediate coagulum formation.
Another method of producing cationically-charged polymer dispersions is transforming a negatively-charged polymer dispersion to a cationically-charged one by adding a cationic surfactant. This process, often referred to as “flipping,” has been exercised commercially for at least 30 years with anionic polystyrene butadiene rubber (SBR) latex. This flipping process is limited to a very narrow range of polymer dispersions and is difficult to exercise with most carboxylated polymer dispersions. U.S. Pat. No. 5,045,576 assigned to Dow Chemical describes a method of flipping a carboxylated poly(styrene-butadiene) latex to produce a cationic polymer dispersion that can be used in asphalt applications. In the flipping process described in U.S. Pat. No. 5,045,576, a large amount of a nonionic emulsifier with a high molecular weight of polyethylene oxide ((EO)m where m>20) is added to the polymer dispersion prior to addition of the cationic emulsifier. The pH of the dispersion is then reduced by adding an aqueous inorganic acid solution.
One disadvantage of the polymer dispersions produced via the described flipping process is that a polymer film prepared from the polymer dispersion is highly sensitive to water, resulting in a low wet tensile strength with a high degree of water adsorption. Another disadvantage of polymer dispersions produced with the described flipping process is that, in asphalt applications, the presence of the surfactants reduces asphalt adhesion to aggregate. This reduction of adhesion causes premature stripping of the asphalt from the aggregate surface by traffic during the lifetime of the pavement.
Another method of producing polymer dispersions, particularly poly(ethylene-vinyl acetate) or polychloroprene dispersions, comprises polymerizing the monomers in the presence of water-soluble, high molecular weight, nonionic polymers. These nonionic polymer molecules adsorb on the surface of polymer particles during the emulsion polymerization process, and act as a protective colloid to maintain the dispersion's stability. However, a disadvantage to using these types of protective colloids is that a polymer film prepared from a polymer dispersion that includes these protective colloids is sensitive to water and displays a high degree of water absorption and a low wet tensile strength. Thus, polymer dispersions produced using these protective colloids suffer from the same problems as polymer dispersions produced using the flipping process. A polychloroprene dispersion produced using these protective colloids (Neoprene from DuPont Elastomers) has been used for various applications since the 1950's.
Polymer dispersions stabilized with a protective colloid by the process described above can be combined with a cationic surfactant to produce a cationic emulsion. However, a disadvantage to producing cationic emulsions in this manner is that only limited types of cationic surfactants can be used to produce the polymer dispersions. Furthermore, the resulting emulsion can actually be too stable, and the excess stability can hinder the ability of the polymer dispersion to cure for certain applications, such as rapid-setting asphalt emulsions.
Because of above discussed limitations in the methods of producing cationic polymer dispersions, it would be desirable to produce a cationic polymer dispersion wherein the resulting dried polymer film is highly water resistant; absorbs very little water, and maintains a high wet tensile strength. It is also desired that these polymer dispersions can be used over a wide range of pH's from acidic to alkaline conditions. Furthermore, it would be desirable to produce a polymer dispersion that can maintain its colloidal stability in the presence of various types of cationic surfactants and cationic polyelectrolytes, such as those typically used for paper making, paper sizing and road paving applications. It would also be desirable to be able to use existing production facilities for anionic polymer dispersions to produce polymer dispersions that can be rendered into cationic polymer dispersions having the desired properties described herein.